In DE 4036734 C1, for example, a spin blowing process and apparatus for producing a spun fleece web are described in which a multiplicity of filaments emerging from the nozzle orifices of a nozzle plate or spinneret, constituted of thermoplastified synthetic resin, are entrained in flat streams of air delivered to the region of the orifice plate by appropriate ducts and slot-like air nozzles, to generate a descending curtain of the filaments and the entraining air through a gap.
This gap which is elongated to extend across the entire working width of the apparatus and of the web, is formed in the spin-blowing head disposed above a continuously moving sieve belt upon which the blowing air/filament mixture descends so that the fibers collect upon the belt to form a mat or fleece which can, if desired, be consolidated between calender rolls or can be recovered as a nonwoven mat or fleece, hereinafter referred to as the web. The fibers which accommodate to form the fleece or web can be collected against the belt by reason of a subatmospheric pressure or suction generated below the belt if desired. The spin blown web which is formed has wide applications as absorbent materials, fabrics for a wide variety of purposes, fillings and the like. The spun-fiber fleece can be produced with a variety of mechanical properties, such as the specific weight and the specific air permeability of the web. The term "specific" is here used to refer to the property per unit area.
In general, it is important that these properties be as homogeneous as possible and it has been found that the uniformity of the specific weight (weight per unit area) and specific air permeability (air permeability per unit area) can depend upon the microstructure of the fibers and, especially, the deviations from homogeneity can be corrected during the process of fabricating the spun fiber fleece or web. In particular, we have determined that factors like the pore size or so-called mesh width of the collected fibers which are dependent upon the microstructure of the fibers and the microstructure with which they unite with one another in the fleece or web, can be influenced positively during the fabrication process.